"Struggles" feels overblown, but it's a factor to be aware of.
Sindel's d1 is 8 frames, whereas most toons have a 7-frame d1. It's not a huge difference, but it does mean there are a few situations where Sindel can't steal her turn with a poke where a 7-frame poke would be just fast enough. Her s1 is quick at 7 frames but very stubby, so she won't be able to punish in some situations with a tight window and moderate pushback. These aren't deal-breakers; you just have to spend time in the lab to learn where she has trouble.
As for coping with pressure, as they say, an ounce of prevention is worth a pound of cure. Sindel out-spaces most of the cast with her buttons, so use that to your advantage to check movement and avoid being cornered. Her float is mostly trash unfortunately, but can also sometimes get you out of a tight spot.
You might want to consider learning Royal Edenian first. It's generally considered her all-around most useful variation, because she gets some tools that specifically help maintain the spacing she depends on and check opponents at any range. Splitting Hairs is flashy and fun, but very susceptible to being zoned out and to opponents who can play lame and force Sindel to come to them. It's still really good in certain MUs tho.
Just my own two cents.
